Nahcolite is an ore consisting primarily of sodium bicarbonate (NaHCO3). There are for instance vast quantities of Nahcolite in the Piceance Creek Basin in Northwestern Colorado, which deposits are in the form of beds and disseminated crystals in the Saline Zone of the Green River formation.
Trona ore is a mineral that contains about 90-95% sodium sesquicarbonate (Na2CO3.NaHCO3.2H2O). A vast deposit of mineral trona is found in southwestern Wyoming near Green River. This deposit includes beds of trona and mixed trona and halite (rock salt or NaCl). By conservative estimates, the major trona beds contain about 75 billion metric tons of ore. The different beds overlap each other and are separated by layers of shale. The quality of the trona varies depending on its particular location in the stratum.
A typical analysis of the trona ore mined in Green River is as follows:
TABLE 1ConstituentWeight PercentNa2CO343.6NaHCO334.5H2O (crystalline and free moisture)15.4NaCl0.01Na2SO40.01Fe2O30.14Organics0.3Insolubles6.3
The sodium sesquicarbonate found in trona ore is a complex salt that is soluble in water and dissolves to yield approximately 5 parts by weight sodium carbonate (Na2CO3) and 4 parts sodium bicarbonate (NaHCO3), as shown in the above analysis. The trona ore is processed to remove the insoluble material, the organic matter and other impurities to recover the valuable alkali contained in the trona.
The most valuable soda value produced from trona is sodium carbonate. Sodium carbonate is one of the largest volume commodities made in the United States. In 1992, trona-based sodium carbonate from Wyoming comprised about 90% of the total U.S. soda ash production. Sodium carbonate finds major use in the glass-making industry and for the production of sodium bicarbonate (also called baking soda), detergents and paper products.
A common method to produce sodium carbonate from trona ore is known as the “monohydrate process”. In that process, crushed trona ore is calcined (i.e., heated) into crude sodium carbonate which is then dissolved in water. The resulting water solution is purified and fed to a crystallizer where pure sodium carbonate monohydrate crystals are crystallized. The monohydrate crystals are separated from the mother liquor and then dried into anhydrous sodium carbonate. A purge waste stream is extracted from the mother liquor and discharged from the process and sent for instance to storage ponds, in order to keep the impurities level at a specified value. The disposal of this purge stream can raise problems, particularly when the trona ore contains high concentrations in impurities, which requires higher flow rates of purge streams.
The soda values can be extracted from the underground deposits by mechanical mining. Such mining is however very expensive and requires large deposits, allowing penetration of humans and large machines. Attempts to reduce the cost of extraction and to exploit deposits having smaller layers of ore have been made, by the use of solution mining methods. For example U.S. Pat. No. 4,636,289 discloses a method for recovering sodium carbonate from trona and other mixtures of sodium carbonate and sodium bicarbonate. In U.S. Pat. No. 4,636,289, sodium hydroxide is produced in electrodialytic cells and used to solution mine the mineral ore. However, this process requires the introduction of sodium sulfates into the acid compartments of the electrodialysers, which appears to be difficult to put into practice in a cost effective and efficient way.
In U.S. Pat. No. 4,344,650, which concerns the recovering of alkali values from underground deposits containing large amounts of impurities, is described a process wherein sodium hydroxide is introduced in a trona deposit, thereby solubilizing a portion of the trona as sodium carbonate, extraction the sodium carbonate and carbonating it into sodium bicarbonate. The sodium bicarbonate is successively crystallized, calcined, dissolved and recrystallized to eliminate the impurities. This process is however complex and requires a lot of energy for the crystallizations and calcination.
The invention aims at producing sodium carbonate and/or sodium bicarbonate from ore minerals, in a simple, economical way, avoiding the large energy consumption of the known processes.